The water-leaving radiance (Lw) and the remote sensing reflectivity (Rrs) are the core parameters of ocean optics. For example, the intrinsic optical quantity, the chlorophyll concentration, the optical shallow water substratum and other parameters can be obtained by inversion from Rrs. Moreover, the on-site measured Lw can be used for ground calibration, authenticity testing, and atmospheric correction of an airborne or spatial sensor.
The spectrum of the on-site measured Lw has been in a 50-year history. There are three measurement methods that have been developed and implemented in the prior art, including an over-the-surface measurement method, an underwater profile measurement method and a water surface floating measurement method. Although these methods are easy to implement in the measurement site, but incapable of directly measuring Lw of the water body. In addition, when Lw is derived by measuring some of the relevant parameters, because there are some uncertain factors in the derivation process, the Lw derived from these methods has a great error.
As described in many studies, it is not possible to accurately determine the reflectivity of a gas-water interface if the sea surface is roughened by waves when Lw is estimated by using the over-the-surface measurement method, so that it is difficult to accurately remove reflected light of the surface, resulting in the introduction of huge uncertain factors in the process of deriving Lw. In the underwater profile measurement method, in order to derive the upward radiance to the water surface, it is necessary to obtain a correct attenuation coefficient by means of precise data processing; in practical applications, it is very difficult to accurately estimate the attenuation coefficient of propagation for highly turbid waters or vertically stratified waters, and therefore, there are also huge uncertain factors in the final derivation of Lw. For the surface floating measurement method, it is necessary to derive the upward radiance at 10-50 cm below the water surface as the upward radiance just at 0 cm below the water surface. Moreover, for the underwater profile measurement method and the surface floating measurement method, it is necessary to subjectively derive the water-leaving radiance Lw from the upward radiance just at 0 cm below the water surface by assuming the refractive index and the cross-sectional reflectivity of water. Therefore, even if each component is measured accurately, a variety of degrees of uncertain factors will still affect the calculation of Lw.
At present, the water surface floating measurement method has been rarely mentioned, but there are representative instruments for the over-the-surface the measurement method and the underwater profile measurement methods. For example, representative observation instruments using the over-the-surface measurement method are HyperSAS apparent spectrum observation devices, and the like manufactured in Canada's Satlantic Company; representative observation instruments using the underwater profile measurement method are ocean optical buoys, and the like.
In summary, regardless of the kind of the observation instrument, the water-leaving radiance cannot be measured accurately as long as any of the above three measurement methods is adopted in the prior art. Therefore, it is necessary to develop a device for observation of an apparent spectrum of a water body based on a new observation method, which makes use of a method of deriving the water-leaving radiance rather than direct measurement, thereby furthest reducing the method defectors, personal errors, device investment and the difficulty in operation and maintenance, and achieving a higher observation accuracy.